Providing the Right Specifications Will Help Ensure Delivery of the Right Equipment
By James Hu, PE, Senior Project Engineer
Providing straightforward specifications for thickener and clarifier drives will help ensure that users receive equipment that meets their performance expectations. It is important to remember that stating requirements vaguely will leave suppliers with a large area for interpretation.
Ask for the Gear You Want
For example, specification used in the water industry reads: "Gear units shall be rated for a minimum life of 100,000 hours with a reliability factor of 95 percent. Continuously running equipment shall have a minimum service factor of 1.5. In this specification, the first requirement is a "minimum life of 100,000 hours." The intent is to define a gear that will last at least 100,000 hours before it fails, but this specification can have at least two different meanings. A gear designer will think of two possible gear failure modes: surface durability failure or pitting of the gear surface, and strength failure or cracking of the gear tooth. To the nonspecialist, the latter mode is what is commonly thought of as failure.
Both modes of failure are calculated when gears are analyzed. The proper gear rating per American Gear Manufacturers Association (AGMA) or ISO standards will be the lower value of either surface durability or strength. However, the example specification makes no requirement for the evaluation of life to be made by any standard at all. Nor does it indicate what is meant by "failure" at the end of 100,000 hours.
This oversight will make a considerable difference. For example, the final gear set in the DBS D30-AF, a common municipal clarifier drive, has the following ratings for 100,000 hours with a 95 percent reliability factor, surface durability of 30,840 Nm, and strength of 62,180 Nm. In this case, the strength rating is more than twice the numerical value of the surface durability rating. A supplier could interpret the specification on the strength of his gear, thus technically meeting the specification with a gear that has a surface durability of only half of what the person specifying intended.
The second requirement in the example states that a service factor of 1.5 be applied. However, the specification fails to define what service factor should be applied. Should it be applied to the catalog rating of the drive, the 100,000 hour gear rating, or the 40,000 hour L10 bearing rating?
Additionally, is this service factor intended to apply to the gearbox bearings or the gears? Typically commercial gearboxes have catalog ratings with 1.0 service factor, which provide an L10 bearing life of 5,000 hours. This 1.0 catalog rating, however, is based on the gearbox bearing life, not the gear life. When a 1.5 service factor is applied for "continuous duty," this is principally to increase bearing life, not gear life. Of course, gear life also increases. Again, lack of clarity in the specification could lead to widely different interpretations.
Prevent Specification Misinterpretations
The solution to this problem is quite simple. The specifications only require the application of a national standard, such as AGMA or ISO for gearing. The expected life rating in hours or cycles should also be stated alongside the L10 life of the gearbox bearings. Calculations to substantiate the gear capacity and life, and the bearing L10 life, should be submitted with any job bid. Specifications must have statements like this to insure provision of proper equipment. “Gear units shall be rated for a minimum life of 100,000 hours per ISO Standard 6336. Bearings shall have a minimum L10 life of 100,000 hours per BS5512 Part 1.” To offer more clarity for a request, calculations must be submitted with a bid package.